From Global to Local-New Insights into Features of Pyrethroid Detoxification in Vector Mosquitoes

Abstract

The threat of mosquito-borne diseases continues to be a problem for public health in subtropical and tropical regions of the world; in response, there has been increased use of adulticidal insecticides, such as pyrethroids, in human habitation areas over the last thirty years. As a result, the prevalence of pyrethroid-resistant genetic markers in natural mosquito populations has increased at an alarming rate. This review details recent advances in the understanding of specific mechanisms associated with pyrethroid resistance, with emphasis on features of insecticide detoxification and the interdependence of multiple cellular pathways. Together, these advances add important context to the understanding of the processes that are selected in resistant mosquitoes. Specifically, before pyrethroids bind to their targets on motoneurons, they must first permeate the outer cuticle and diffuse to inner tissues. Resistant mosquitoes have evolved detoxification mechanisms that rely on cytochrome P450s (CYP), esterases, carboxyesterases, and other oxidation/reduction (redox) components to effectively detoxify pyrethroids to nontoxic breakdown products that are then excreted. Enhanced resistance mechanisms have evolved to include alteration of gene copy number, transcriptional and post-transcriptional regulation of gene expression, as well as changes to cellular signaling mechanisms. Here, we outline the variety of ways in which detoxification has been selected in various mosquito populations, as well as key gene categories involved. Pathways associated with potential new genes of interest are proposed. Consideration of multiple cellular pathways could provide opportunities for development of new insecticides.

Keywords: Aedes; Anopheles; Culex; deltamethrin; detoxification; insecticide; metabolic resistance; mosquito; permethrin; pyrethroid.

Figure 1

Three major pyrethroid resistance mechanisms. (A) At the target site, pyrethroid molecules (gold spheres) bind voltage-gated sodium channel (VGSC) (aqua) to depolarize motoneurons and cause paralysis. Target site mutations were subsequently selected in resistant populations. (B) Biochemical effectors detoxify pyrethroids to nontoxic forms, e.g., phenoxybenzoic acid (PBAc), that are excreted (Figure 2). Pyrethroid is indicated in gold spheres; cytochrome P450 (CYP) proteins are indicated in green; carboxylesterases (COEs) are indicated in pink; rejuvenation of CYP (from blue oxidized to reduced green form) by nicotinamide adenine dinucleotide phosphate (NADPH) cytochrome P450-reductase (CPR) [45]. (C) Cuticular thickening and/or modifications are also features of resistance. Figure prepared using Biorender, with permission (Biorender.com, accessed on 22 March 2021). Chemical structures were drawn using Marvin JS (v. 17.4.3, chemaxon.com, accessed on 22 March 2021).

Figure 2

Pyrethroid detoxification cascade is mediated by carboxylesterases (COEs), cytochrome P450s (CYP6, CYP9) and other oxidation/reduction proteins. Permethrin is converted to PBAlc (phenoxybenzyl alcohol) by COEs and/or CYP9/CYP6 [45]. The metabolic intermediate PBAlc is converted to PBAld (3-phenoxybenzaldehyde). Aldehyde dehydrogenase is a demonstrated oxidizer of PBacid [54]. PBAc is nontoxic in larvae [45]. Recent genetic association evidence also implicated senecionine N-oxygenase as a possible player in pyrethroid resistance [31,89]. Proposed involvement of aldehyde dehydrogenase or senecionine N-oxygenase on right. Oxidized groups are shown in red in chemical structures. Figure prepared using Biorender, with permission (Biorender.com, accessed on 22 March 2021).

Figure 3

Signaling mechanisms contribute to pyrethroid resistance. Activation of the signaling mechanism could occur as part of the wound response [118,119,123]. GPCR signaling and subsequent upregulation of detoxification enzyme expression has been described in Culex spp. [34,117]. Increased resistance was observed in the presence of elevated cyclic adenosine-monophosphate (cAMP). Effectors in red font are genetically associated with pyrethroid resistance in natural Ae. aegypti collections from Mexico [31]. Atypical protein kinase C (aPKC), which helps maintain cell polarity, could activate NF-κb upregulation of resistance genes [122]. Figure prepared using Biorender, with permission (Biorender.com, accessed on 22 March 2021).